Q.In many dicot plants, the lumen of the xylem vessels is blocked by many balloon like in-growths from the neighbouring parenchymatous cells. These balloon like structures are called ________.

As a tree's secondary xylem (wood) ages, it passes through two functionally and visually distinct zones. Sap wood, also called alburnum, is the pale-coloured, physiologically active outer zone - the youngest, most recently formed wood, whose vessels remain open and continue conducting water and dissolved minerals up the plant. Heart wood, also called duramen, is the dark-coloured, older, inner zone, formed as sapwood ages and stops functioning in water conduction; the lumens of its xylem vessels become blocked by tyloses - balloon-like ingrowths that bulge in from neighbouring living parenchyma cells through pits in the vessel wall - and the wood itself becomes infiltrated with tannins, resins, gums and similar organic substances, which is precisely what darkens heartwood and makes it markedly more resistant to decay, fungal attack and insect damage than sapwood. …